Steel strapping and strip can have various tensile strengths and ductility depending upon the end use of the strapping or strip.
Flat steel strapping has many uses in holding together articles such as cartons, machinery to pallets, bricks, cotton, lumber to a railroad car, containers, crates, goods and the like. The thinness of the strapping permits it to slide around corners of the article without applying excess stress to the article. The lack of applying excess stress to the article is particularly important when the article is relatively readily deformable, e.g., cardboard, wood or the like, and permits a greater tension to be applied to the strapping than if a thicker product, e.g., wire, was used. The width of the strapping minimizes indentation of the corners over time which results in the strapping retaining the tension applied thereto because the shape of the strapped article is not deformed over time by the strapping. In contrast, wire is often round and has a greater thickness that increases the stressing and deformation of the article. Wire also has a thinner width that can cause indentation of the corners over time and result in a loss of tension. The flat ends of the flat strapping are easier to join together than the round ends of round wire. The flat strapping can have a strength that is equivalent to that of round wire but the strapping is much thinner than the wire. For example, 0.75 inch wide by 0.025 inch thick strapping has about the same strength as high tensile strength round wire having a diameter of 0.162 inches. Thus, the strapping has the advantage of being about 4.5 times wider and almost 6.5 times thinner than the wire.
The physical properties of the steel strapping are selected depending upon the use for the strapping.
The strapping used to hold a protective carton over machinery secured to a pallet does not experience a great load and therefore can be a lower strength strapping.
The strapping used to secure the machinery to the pallet must have a strength commensurate with the weight of the machinery because the weight of the machinery will determine the force exerted upon the strapping. The ductility, as measured by the elongation, of the strapping can be important because the strapping must be pulled tightly around the machinery to conform to the shape of the machinery. A shape having sharp corners requires a strapping having a greater ductility than a shape having round corners. Thus, the strapping may have to be strong and ducthe. Typically, standard high tensile steel strapping having a tensile strength of about 125,000 to about 145,000 pounds per square inch (psi) and an elongation in a 6 inch specimen of about 5 to about 15% that meets the Association of American Railroads (AAR) and the American Society for Testing and Material (ASTM) specifications is used to secure machinery.
The strapping used to form a cube of bricks must be strong enough to bear the load exerted on it by the bricks. The strapping must also have a high degree of ductility as the strapping must slide and bend around the sharp corners during tensioning of the strapping to form the cube. Thus, the strapping must be strong and ducthe. The strapping can be made from a dual phase steel.
The strapping used to produce cotton bales must have adequate strength to withstand the force exerted thereon when the bale is released from compression in the baling machine. The cotton bale strapping requires relatively less ductility as compared to strapping used to produce cubes of bricks because the cotton bales typically have a round cross-section. A round cross-section does not have sharp corners over which the strapping must be pulled during tensioning. Strapping having a tensile strength of about 200,000 psi is suitable for the purpose with no requirement for elongation.
Loads of material, e.g. lumber, can be secured to railroad flat cars using strapping. The lumber is first bundled together using strapping, which are referred to as "package bands", that have a cross-section in the range of about 1/2.times.0.020 to about 1 1/4.times.0.031 inches and that can have a strength of about 1,200 to about 4,500 lbs. Bundles are then secured to the railroad car using strapping, which are referred to as "securement bands", that have a cross-section in the range of about `1/4.times.0.031 to about 2.times.0.065 inches and that can have a strength of more than about 13,800 lbs. The securement bands must withstand the load exerted thereon by numerous bundles and have a high degree of ductility to slide around sharp corners of the bundles and of rectangular steel stake brackets on the sides of the railroad car about which the securement bands are looped and secured.
High tensile strapping is conventionally produced by slitting full hard, cold rolled sheet steel to the cross-sectional size of the strapping followed by heating to an elevated temperature. The strapping is usually quenched by immersion in a molten lead bath. This immersion of the hot steel in the molten lead bath is referred to as patenting. The range of ductility afforded by the ability to vary the quench temperature in patenting is appropriate for most steel strapping applications. Metallurgists believe the resulting grain structure is a bainite or a mixed structure of pearlite, ferrite and martensite.
Slitting results in the production of square, sharp corners and burrs that can tear the article that is strapped and that can be harmful to people who come into contact with the strapping. The sharp corners and burrs also undesirable enhances transverse crack propagation and reduce the fatigue life of the strapping which reduces the toughness of the strapping as compared to strapping having round edges. The customary burr removal operation of rolling or flattening imparts cold work stresses on the strapping edge that further exacerbate the cracking problem. Strapping produced by slitting requires numerous finishing steps after cold working, e.g., heat treating and quenching, to improve strength and ductility. These finishing steps increase the cost of the strapping.
The slitting step is often a batch step. A batch step is usually less desirable than a continuous step for the manufacture of a large volume of strapping because continuous processes are more cost efficient. In a batch step the desired amount of steel enters the step and then the step is performed on the steel. In a continuous step the steel moves continuously through the step as the step is being performed on the steel. A batch step cannot be used in a continuous process.
High tensile strength strapping can be produced from sheet steel by the steps of hot rolling, cold rolling, and slitting followed by an electric resistance heat treating process. Unfortunately, this process requires numerous steps including a slitting of the hot and cold rolled sheet that removes the edges and therefore results in a loss of finished product. Also, this process for making high tensile strapping requires slitting and therefore has the problems associated with slitting.
The strapping produced by a conventional process can have physical properties similar to or exceeding those of a mixed structure of pearlite, ferrite and martensite, which is typical of a patented or lead quenched structure by using a separate heating step followed by immediate quenching in lead to 800.degree. to 950.degree. F. followed by quenching in water to ambient temperature, e.g., about 60.degree. to about 90.degree. F. Unfortunately, additional process steps are utilized.
The strip is used to form, as by stamping, metal parts, e.g., lawn mower blades, scissor blades, and the like, that are subsequently heat treated to achieve the desired physical properties of the finished part. The strip must have the proper ductility and tensile strength to permit parts to be formed.
The strip produced by specialty steel strip manufacturers requires multiple steps including a slitting step. Generally, the manufacturer starts with a steel coil having a relatively high carbon content. The coil is formed while the steel is still hot which causes a nonuniform grain structure throughout the coil due to a differential cooling rate of the coil. In the first processing step the coil of steel is subjected to sorbitic annealing to obtain a uniform grain structure. The surface of the steel is then subjected to pickling by immersion into an acidic or alkaline solution to remove the oxide or scale coating thereon. Pickling is performed because the surface has a great effect on the next step which is cold rolling. After about a 50% cold reduction by cold rolling, the steel goes into a furnace for another intermediate annealing prior to the final cold reduction. Then, the cold rolled steel is slit to the final width of the strip. The end product is a cold worked grain structure from an annealed starting steel.
Unfortunately, the specialty steel manufacturer must slit the steel which introduces the previously discussed problems. Also, the higher carbon content steels cannot be made into the strip without intermediate annealing due to the brittleness introduced by cold rolling. Annealing is undesirable because it requires heating of the cold steel with its attendant cost. Also, annealing can be a batch step and therefore cannot be utilized in a continuous process.
Often, the strapping and the strip cannot be made on the same line due to the differences in the starting materials, in the heat treatment and the like. The owner of the line is therefore limited to producing only the strapping or the strip and cannot satisfy both markets.
There is a need for a method of manufacturing flat, directed temper steel strapping and strip that does not utilize a slitting step to reduce the steel to the cross-sectional size of the finished strapping or strip, that is preferably a continuous process, or that does not require reheating the steel after the steel has been cold worked. The present invention satisfies at least one of these needs.